1. Field of the Invention
The present invention relates generally to a NAND flash memory device, and more specifically, the present invention relates to a NAND flash memory device having a security redundancy block, and to a method for repairing a NAND flash memory device have a defective security block.
A claim of priority is made to Korean Patent Application No. 2005-03842, filed on Jan. 14, 2005, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
Flash memory devices, called “Flash Electrically Erasable Programmable Read Only Memory” (Flash EEPROM) or “E2PROM”, are non-volatile memories which maintain stored data even in the absence of supplied power.
Generally, a NAND flash memory device has a plurality of memory cells that are configured in a string structure. The memory cell array of the NAND flash memory device includes a plurality of memory blocks, and each of the blocks defines a plurality of pages formed by a plurality of memory cells sharing one word line. For example, in conventional NAND flash memory devices, the cell array is formed by 1,024 or 2,048 blocks, each of the blocks has 16, 32, or 64 pages, and 512 bytes or 1 kilobyte of memory cells are connected to each of the pages. In NAND memory devices, erase operations are performed on a block-by-block basis, while read and write operations are performed on a page-by-page basis.
It is possible for a fabricated memory cell of a NAND flash memory to suffer an operational failure. A block having at least one failed memory cell is considered a “failed block.” If the number of failed blocks in a NAND flash memory device is more than a threshold value, e.g., more than five, then the NAND flash memory device is determined to be defective. On the other hand, if the number of failed blocks of the NAND flash memory device is fewer than the threshold number of failed blocks, the device is configured to utilize redundant blocks to replace the bad blocks.
However, conventional NAND flash memory devices are not capable of utilizing redundant blocks to replace faulty security blocks. The security blocks are specially configured to store security-related data such as the serial number of device manufacturer, the date of manufacture, and other security-related information. If the security block fails, the NAND flash memory device can not be repaired and is considered defective.